The invention relates to a method of preparing metal powders, particularly iron powder.
The term xe2x80x9cmetal powderxe2x80x9d covers both pure metal powders and powders of metal alloys.
More precisely, the invention is concerned with a method of preparing metal powders, in particular, iron powder by thermal reduction of transition metal or rare earth metal oxalates or oxides that result from the decomposition of transition metal or rare earth metal oxalates, for example, iron oxalates or iron oxides resulting from iron oxalates, these oxalates or oxides being constituted by particles with needle-like morphology having a size, more precisely a length that is specific.
The metal powder, for example the prepared iron powder has a spongy and filament-like structure that equals or indeed exceeds the specifications current for such a metal powder, for example such an iron powder, which makes it particularly suitable for numerous uses, in particular, for use in heating compositions for thermo-piles, when they make use of iron powder.
In addition, the invention relates to metal compacts, unstoved or annealed, obtained by application of compacting pressure to the metal powders according to the invention, possibly accompanied by partial sintering.
The invention relates to numerous applications or uses that require the use through pressing of one or more metal powders, pure or alloyed, which may or may not be associated later with other components enclosed in the porosity of the compact.
Among these applications or uses, one may mention, for example, the manufacture of pyrotechnic mixtures comprising iron powder according to the invention and an oxidizing agent, porous electrodes used notably in electrical energy accumulators, such as electro-chemical Nixe2x80x94Cd cells or Ni-metal hydride cells etc., soft or hard magnetic materials, such as armatures for electric motors, and permanent magnets, catalyst supports, in which a catalytic powder is enclosed within a porous metal structure, filters that are able to make use of the ferro-magnetic properties of certain metals for magnetic separation and, more generally, mechanical components of complex shape which can be produced by a simple pressing operation, avoiding having to carry out complicated machining operations.
The technological field of the invention can therefore be defined as that of metal powders and their preparation, as well as unstoved or annealed metal compacts prepared by pressing these powders, this pressing operation being possibly accompanied by partial sintering.
Such compacts find uses in numerous sectors of industry, in which they must have high mechanical strength.
The compacts prepared from known metal powders have mechanical strengths that are markedly inadequate.
In a preferred manner, which is non-limitative, the invention is more particularly set in the field of thermo-piles.
In particular, the invention relates to a heating pyrotechnic composition comprising the metal powder, in particular iron powder, according to the invention, as well as the thermo-pile comprising this heating pyrotechnic composition.
Thermo-piles are non-rechargeable cells, inert before initiation, which can be stored without any maintenance, sometimes for more than 20 years while remaining usable at any time, with a response time that can be less than three tenths of a second.
Thermo-piles find increasing use in all fields where there is a need for immediately available energy, in a reliable manner, even after a very long storage time.
Thermo-piles are thus mainly used in the aeronautics and space industries and also in all emergency services that require such a source of energy for example, in the nuclear industry, the oil industry and commercial and warehousing buildings etc.
A compacted composition of iron/potassium perchlorate is the traditional heating pyrotechnic composition for thermo-piles.
In effect, this composition has shown a clear superiority in comparison with so-called xe2x80x9cpaperxe2x80x9d zirconium/barium chromate used in the past as a heating pyrotechnic composition for thermo-piles.
The performance of these thermo-piles is directly linked to the properties of this composition and in particular to the microstructure of the iron powder.
The iron powder most commonly used up to now in iron/potassium perchlorate heating compositions for thermo-piles is the powder sold under the name of NX-1000 powder by the Company PFIZER METAL and Composite Products of Walkingford Connecticut, USA or by the Company AMETEKO Specialty Metal Products Division.
This iron powder meets all the following specifications which an iron powder for thermo-piles must fulfill: a sponge-like structure, a total iron content greater than 95%, an elemental iron content greater than 89%, a specific surface Sw (m2/g) greater than 0.5 m2/g, a loss with hydrogen less than 3%, a Scott density of from 0.8 to 1.15, a Fischer sub-granulometry of from 1.5 to 3.5 xcexcm, a granulometry in which more than 70% of the particles pass a sieve with a 44 xcexcm opening and less than 1% of the particles are retained on a sieve with a 150 xcexcm opening, a xe2x80x9cGreen forcexe2x80x9d or compact resistance (determined in accordance with standard ASTM B312-56T) greater than 35 MPa for a forming pressure of 276 MPa.
Numerous methods have been described in the literature to prepare iron powder, but none of these methods permits the preparation of iron powder that can be used in pyrotechnic heating compositions for thermo-piles and that can meet the specifications given above.
Document U.S. Pat. No. 4,414,021 relates to a method of preparing iron powder suitable for use in iron-potassium perchlorate pyrotechnic heating mixtures for thermo-piles.
This method comprises the preparation of a homogeneous dense precipitate of iron hydroxide by precipitation from an aqueous solution of a ferric salt, formic and sulfuric acids, ammonium hydroxide and urea as precipitation agent.
The dense precipitate obtained is then reduced by hydrogen at 650 to 900xc2x0 C. for 0.5 to 2 hours to prepare the iron powder which has a spongy structure and which meets the specifications given above.
The precipitate obtained in the first stage of the method is an iron oxyhydroxide which generally has a formula FeOOH. In effect, if ferric sulfate is used in the aqueous solution, the salt formed is Fe3(SO4)2(OH)5.2H2O which subsequently gives an iron powder having a sulfur content that is excessive for use in a thermo-pile.
The shape and the morphology of the powder produced from the precipitate has not been made clear.
In example 5, this document studies the reduction of various iron based compounds by hydrogen.
Among the twelve compounds tested in fifty tests, only five, following reduction by hydrogen, gave iron powders suitable for use in a thermo-pile, that is to say, powders having properties equivalent to iron powder NX-1000 from PFIZER(copyright) (AMETEK(copyright)). Apart from the precipitates already mentioned, these compounds are various ferric nitrates and oxides.
Ferrous oxalate, the morphology of which is not described, has also been subjected to reduction by hydrogen, in four different tests. This compound gives an iron powder with a spongy structure, but according to the inventors, this iron powder prepared from ferrous oxalate is not suitable for use in a thermo-pile and is not analogous to the NX-1000 powder from PFIZER(copyright) (AMETEK(copyright)).
The method described in the document U.S. Pat. No. 4,414,021, though it apparently enables one to prepare an iron powder that meets the specifications given above, still has numerous disadvantages, for example, since no specification is given concerning the morphology of xe2x80x9cgoodxe2x80x9d precursors, no controls on them can be carried out before the reduction stage, which can lead to random results in relation to the properties of the iron powders produced according to the method of document U.S. Pat. No. 4,414,021.
Similarly, as has already been mentioned above, in the more general case of the preparation of metal powders, the metal powders prepared at present by the known methods give, when pressed, compacts, the strength or mechanical resistance of which is markedly inadequate for most applications.
Therefore there is an unsatisfied need for a method that permits the preparation of a metal powder which through the pressing of compacts, gives improved resistance or mechanical strength, the other properties of such a powder additionally being within satisfactory ranges.
More precisely, there is an unsatisfied need for a method that permits the preparation of iron powder that meets at least the specifications given above and is suitable as a constituent of pyrotechnic heating compositions.
In addition, there exists a need for a method that permits the preparation of such a metal powder at high yield. This method must finally be reliable, reproducible, easy to carry out, include a limited number of steps and permit the preparation within a short period of time of large quantities of metal powder, for example, iron powder.
The object of the invention is to provide a method of preparation of a metal powder, in particular, an iron powder, that responds to the group of needs, among others, given above for such a method.
The object of the invention is also to provide a method that does not have the drawbacks, disadvantages and limitations of the methods of the prior art and which resolves the problems posed by these methods.
This object and others, are achieved, conforming to the invention, by a method of preparing a metal powder or a metal alloy powder comprising the following steps:
preparation of a simple or mixed oxalate of one or more metals made up of particles with a needle-like morphology, the mean acicularity ratio (length/diameter) being between 4 and 20, and the length from 5 to 10 xcexcm.
reduction of said metal oxalate by treatment with gaseous hydrogen.
According to a variant of the method of the invention, in the second stage, instead of proceeding with the reduction of the metal oxalate by treatment with gaseous hydrogen, the decomposition of said oxalate to an oxide takes place in air. Then reduction of said oxide is carried out to obtain the metal or metal alloy powder that is sought.
The decomposition in air is, in this case, generally carried out at a temperature of from 250xc2x0 C. to 600xc2x0 C.
The metal is chosen preferably from among the transition metals and the rare earth metals.
Preferably, the metal is chosen from among iron, cobalt, nickel, . . . .
The method according to the invention is particularly suitable for the preparation of iron powders. In this case the metal oxalate is ferrous oxalate.
The gaseous hydrogen may be pure or gaseous hydrogen or gaseous hydrogen diluted in a neutral gas, such as nitrogen.
Because of the specific morphology of the oxalate precursor, it""s specific size (length), and, in addition, the specific reduction conditions, a metal powder is obtained that meets the needs given above, and in particular, an iron powder which has a spongy and filament-like microstructure which makes it suitable for use in a thermo-pile.
On the other hand, the iron powder prepared by the method of the invention fulfills the conditions and specifications for such an iron powder and is even shown to be superior in most of its characteristics and properties to the known powder sold by the company PFIZER(copyright) (AMETEK(copyright)) under the name NX-1000.
In a general way, the particles of metal powders prepared by the method of the invention have, in a surprising manner, the property of coming together to form a coherent porous solid with high mechanical strength, under the effect of compacting pressure.
This property results from the entanglement of the particles with a spongy and filament-like structure, that is characteristic of the powders which are a subject of this invention.
Hence, by way of example, if the mechanical resistance of the compacts pressed under given conditions, for example 292 MPa, is measured by the well-known three-point bending test, on parallelepiped test pieces of standardized dimensions, it is possible to evaluate the effect of the morphology of the particles on this mechanical property.
It can then be shown that, for pure iron powders, test pieces produced by pressing spherical or polygonal particles with smooth surfaces rupture at pressures of from 10 to 15 MPa, while if the pressed particles have a spongy and filament-like structure, characteristic of the invention, they can resist more than 50 or indeed 60 MPa.
Advantageously, the metal oxalate, for example, ferrous oxalate made up of particles with a needle-like morphology with an acicularity ratio between 4 and 20, and a length of from 5 to 10 xcexcm, is produced by mixing a solution of a metal salt, for example, a ferrous salt and a solution of an oxalic compound, and precipitating the metal oxalate, for example, ferrous oxalate from said mixture.
The preparation of the metal oxalate, for example, ferrous oxalate is carried out by a precipitation that is easy to carry out with readily available starting products which are of moderate cost.
Advantageously, the reduction of the metal oxalate, for example, the prepared ferrous oxalate, is carried out by treatment with gaseous hydrogen at a moderate temperature.
It has been shown that a temperature of from 500 to 700xc2x0 C., preferably from 520 to 620xc2x0 C., is particularly suitable.
Another subject of the invention is the metal powder, in particular the iron powder, made up of spongy and filament-like particles capable of being obtained by the method described above.
Preferably, said particles have a size between 10 and 50 xcexcm.
Another subject of the invention, is a new product, which is an iron powder with a spongy and filament-like microstructure which has the following properties:
total iron content: greater than or equal to 98%;
elemental iron content: greater than or equal to 95.7%;
loss with hydrogen: less than or equal to 1.30%;
specific surface: greater than or equal to 0.50 m2/g;
sub Fischer granulometry: 3.25 to 3.5 xcexcm
granulometry obtained by sieving
more than 70% of particles having a size less than 45 xcexcm;
less than 1% of particles with a size greater than 150 xcexcm
Green force (resistance of the raw compact) for a forming pressure of 276 MPa: greater or equal to 50 MPa.
The iron powder according to the invention conforms to and broadly goes beyond the specifications mentioned above and is superior to the powder NX 1000 AMETEK(copyright), notably with regard to its elemental iron content, the loss with hydrogen, and the green force.
The iron powder according to the invention can be used in numerous fields of industry.
Depending on the use, it can be used alone, as it is, or it can be mixed with other constituents to form a composition.
In addition, the invention relates to compacts capable of being obtained by pressing at least one metal or alloy powder according to the invention.
It has already been mentioned above that these compacts, because of the specific structure of the powder according to the invention, have remarkably improved mechanical strength properties.
These compacts are capable of being obtained by pressing a single pure metal powder or an alloy powder or by pressing several powders, each of these powders being a pure metal powder or an alloy powder.
One or more additional compounds can be additionally added to said pure metal or alloy powder or powders, these compounds then being enclosed within the porosity of the compact; these compounds may be, for example, solids or liquids. Among these compounds, one may mention, for example, ceramics, ionic compounds, hydrides, oxides, ferro-magnetic metals, hydroxides, aliphatic or cyclic organic liquid compounds.
The pressing, carried out for the purpose of obtaining the compacts according to the invention, is generally carried out at a pressure, referred to as the compaction pressure, of from 50 to 500 MPa, preferably, from 100 to 300 MPa.
However, it has been demonstrated that, according to the invention, the mechanical strength (R) of the compacts according to the inventionxe2x80x94which is already much higher than that of compacts prepared from powders of the prior artxe2x80x94could be further significantly improved by using higher compacting pressures, namely a compacting pressure of from 500 to 1000 MPa, and/or by additionally carrying out a partial sintering during or after the pressing operation.
This sintering is preferably a sintering carried out under a reducing atmosphere, for example, an atmosphere made up of hydrogen, nitrogen monoxide, or a hydrogen-nitrogen mixture, or under a neutral atmosphere, for example, an atmosphere of nitrogen, a noble gas, such as argon or one of their mixtures.
The partial sintering is generally carried out at a moderate temperature, for example between 400 and 600xc2x0 C., which only very slightly modifies the porosity of the compacts and enables one to preserve, if necessary, a porosity, for example, greater than 30%, while increasing the mechanical strength.
By way of example, the pressing at 500 MPa of iron powders prepared by the method of the invention gives compacts whose mechanical strength reaches 100 MPa.
Furthermore, a compact for which R is equal to 60 MPa sees its mechanical strength increased to 220 MPa, after partial sintering carried out at 450xc2x0 C. under pure hydrogen.
The powder according to the invention and the compacts according to the invention find uses in numerous fields of technology as has already been mentioned above.
The invention also relates to the use of the powders or of the compacts described above.
The invention thus relates to the use of the metal powder according to the invention to prepare soft or hard ferro-magnetic materials, in particular, filters using the ferro-magnetic properties of one or more metals to carry out ferro-magnetic separations, or to manufacture the armatures for electric motors intended for the automobile and domestic electrical appliance sectors, or to produce permanent magnets.
In these applications, the metal powder is generally chosen from among the ferro-magnetic metal powders, such as iron, cobalt, nickel and their alloys.
The invention also deals with the use of the metal powder according to the invention to prepare porous electrodes, notably used for electrical energy accumulators, such as nickel-cadmium electrochemical cells, or nickel-metal hydride cells.
The invention also relates to the use of the metal powder according to the invention to prepare catalyst supports, in which a catalyst powder is enclosed within the porous metal structure.
In the general way, the invention relates to the use of the metal powder according to the invention to prepare mechanical components of complex shape, which may be produced by a simple pressing operation, avoiding complex machining operations.
In other words, the invention relates to the use of metal powders according to the invention in powder metallurgy.
A preferred use of the metal powders according to the invention and in particular iron powder is use in heating pyrotechnic compositions particularly for thermo-piles.
Therefore, the invention also relates to a heating pyrotechnic composition that includes the iron powder according to the invention.
In such a composition, the iron powder is generally associated with a powerful oxidizing agent chosen from among for example potassium perchlorate, lead dioxide, tungsten oxide, iron oxide etc., in a proportion by weight of from 80 to 88% of iron powder to 12 to 20% of oxidizing agent.
In the case of pyrotechnic compositions for thermo-piles, the oxidizing agent is generally potassium perchlorate and the preferred compositions by weight are generally the following:
84% iron powder and 16% KClO4;
83% iron powder and 17% KClO4.
In order to be used in a thermo-pile, this heating pyrotechnic composition is generally pressed, xe2x80x9ctablettedxe2x80x9d or conpacted so as to obtain tablets the apparent density of which is preferably from 3.2 to 3.8.
The invention is also concerned with such tablets.
Finally, the invention also relates to a thermo-pile which includes a tabletted heating pyrotechnic composition or tablets as described above.